The present invention relates to methods and compositions for viscosifying fluids. More particularly, the present invention relates to networking agents, viscosified treatment fluids, and methods of using these compositions in applications wherein viscosified treatment fluids may be used.
Many industrial applications require viscosified fluids or “treatment fluids.” For instance, the upstream energy industry uses treatment fluids in a variety of production and stimulation operations. For example, a treatment fluid may be used to drill a borehole in a subterranean formation, to stimulate a well bore in a subterranean formation, or to clean up a well bore in a subterranean formation, as well as for numerous other purposes. As used herein, “treatment fluid” refers to any fluid that may be used in a subterranean application in conjunction with a desired function and/or for a desired purpose. The term “treatment fluid” does not imply any particular action by or with the fluid. Oftentimes treatment fluids used in subterranean applications are viscosified. While such fluids may be used for many purposes, one such purpose is to transport solid particulates such as proppant or gravel. Treatment fluids generally have a viscosity that is sufficiently high to suspend particulates for a desired period of time, to transfer hydraulic pressure, and/or to prevent undesired leak-off of fluids into the formation.
Treatment fluids that are used in subterranean operations generally are aqueous-based fluids that comprise a gelling agent. These gelling agents may comprise biopolymers or synthetic polymers. Some common gelling agents include, e.g., galactomannan gums, cellulose derivatives, and other polysaccharides.
The viscosity of a treatment fluid containing a gelling agent may be increased by crosslinking at least some of the gelling agent molecules with a crosslinking agent that may be added to the treatment fluid. Typical crosslinking agents generally comprise a metal, transition metal, or metalloid, collectively referred to herein as “metal(s).” Examples include boron, aluminum, antimony, zirconium, magnesium, or titanium. Under the appropriate conditions (e.g., pH and temperature), the crosslinks that form between gelling agent molecules may increase the viscosity of a treatment fluid.
The chemical nature of any resultant crosslinks, in part, determines the stability and Theological properties of the treatment fluid and, oftentimes, the applications to which the treatment fluid may be put. For example, boron crosslinking agents are frequently used in treatment fluids and are compatible with a number of gelling agents. But boron crosslinking agents are typically limited to use in environments that have a pH of about 8 and above and a temperature below about 300° F. This pH requirement may be problematic because, inter alia, it may preclude the use of seawater in the treatment fluid or the use of the treatment fluid in an offshore environment. Similarly, treatment fluids comprising gelling agents that are crosslinked with boron may suffer from thermal instability at certain elevated temperatures like those frequently encountered in some subterranean operations. In addition, boron crosslinking agents often react with additives commonly added to treatment fluids, e.g., glycols (such as ethylene or propylene glycol) and alcohols (such as methanol). To overcome this propensity, boron crosslinking agents are typically added in excess to treatment fluids, which may increase the environmental footprint and the costs associated with the treatment fluid.
Crosslinking agents that use metals other than boron, such as zirconium and titanium, are also frequently used in treatment fluids. These crosslinking agents generally form crosslinks that are more stable than those formed by boron crosslinking agents. Although treatment fluids that are crosslinked with non-boron crosslinking agents are more stable, they may be more difficult to break, thus making recovery of the fluid from the well bore more difficult.